Militants fighting for the Islamic State in Syria and Iraq are being led by a highly secretive group of strategists and officials that were once senior figures in Saddam Hussein's army.

Despite thousands of foreign fighters flocking to join the Sunni extremist group and starring in their propaganda videos, ISIS' leadership is dominated by ex-members of the late Iraqi dictator's military.

Almost all of the regional commanders appointed by ISIS' leader and self-declared caliph, Abu Bakr al-Baghdadi, played prominent roles in the Baathist army before the 2003 invasion of Iraq and became involved with ISIS either in the resulting insurgency or after the dictator's 2006 execution.

Influence: Despite thousands of foreign fighters flocking to join the Sunni extremist group and starring in their propaganda videos, ISIS' leadership is dominated by ex-members of the military of Saddam Hussein (pictured)

Militants: Almost all the highly secretive officials overseeing the day-to-day running of the Islamic State (pictured) are former members of Saddam Hussein's Baathist army

One of the most prominent former Iraqi Army generals within ISIS was Abu Muslim al-Turkmani (left) who led the terrorists' operations in Iraq until he was killed in an American airstrike last November. Abu Ayman al-Iraqi (right), a former colonel in Iraqi Air Force intelligence now plays a leading role in ISIS' military council

One former Syrian rebel who became an ISIS regional commander told the Washington Post that he found that almost all of the highly secretive officials overseeing all aspects to the terror group's day-to-day business were ex members of Hussein's Baathist army.

During one meeting in which he dared to disagree with fellow commanders at an ISIS meeting, the man calling himself Abu Hamza was placed under arrest on the orders of a masked man who had sat silently and almost unnoticed at the side of the room, listening and taking notes.

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Although the enigmatic commander's real name was never revealed to him, Hamza said that he later discovered that the man had been an intelligence officer for the Iraqi army before joining ISIS.

The same went for the other ISIS commanders Hamza met, who either used code names or no names at all in the hope of concealing their past as members of Hussein's army.

As well as their extensive military expertise, another key element of the commanders' appointments has been the fact the ex-Baathists have extensive contacts among oil smugglers thanks to Hussein's efforts to avoid international sanctions in the 1990s.

Crucially, oil smuggling is considered the most lucrative aspect of ISIS' £2 billion-a-year income.

Leader: Almost all of the regional commanders appointed by ISIS' leader and self-declared caliph, Abu Bakr al-Baghdadi (pictured), played prominent roles in the Baathist army before the 2003 invasion of Iraq

Experience: Former Iraqi Army colonel Haji Bakr (left) was al-Baghdadi's closest advisor and the overall head of his military council until his execution at the hands of Syrian rebels in January 2014. Pictured right is Abu Ahmad al-Alwani - a former Baathist soldier and now an ISIS military council member

The former Iraqi Army generals' connections with oil smugglers are crucial to ISIS' £2 billion-a-year income

Middle East analysts and experts believe one key reason why so many former Iraqi army commanders have joined ISIS is due to the de-Baathification law brought in L.­ Paul Bremer - the American who served as temporary head of the Iraqi state following the ousting of Hussein.

The law effectively meant that 400,000 members of the defeated Iraqi army could no longer be employed in government roles and were barred from drawing their military pensions.

Many of these highly-trained commanders joined the insurgency in the country in the mid-2000s, which was dominated by ISIS who at the time were known by the name Al Qaeda in Iraq.

When al-Baghdadi took control of the group in 2010, many of these commanders were given prominent leadership roles in the hope they might win the support of the Sunni tribes who dominate Western Iraq and who have been largely ignored by the country's Shia-led Baghdad government.

The process had, however been started by al-Baghdadi's predecessor Abu Omar al-Baghdadi who was himself a former Iraqi Army officer and therefore more trusted by the military than the Jordanian criminal, former gangster and ex-alcoholic who founded ISIS, Abu Musab al-Zarqawi.

Past: The process of giving former Iraqi commanders senior roles was started by Abu Bakr al-Baghdadi's predecessor Abu Omar al-Baghdadi (left) who was a former Iraqi Army officer and therefore more trusted than the Jordanian criminal, former gangster and ex-alcoholic who founded ISIS, Abu Musab al-Zarqawi (right)

Heartbreaking: The highest estimate for the number of recruits killed in the Speicher massacre is about 1,700

Discovery: The graves were found last Wednesday on the late Saddam Hussein's presidential compound

One of the most prominent former Iraqi Army generals within ISIS was Fadel Ahmad Abdullah al-Hiyali, who used the code name Abu Muslim al-Turkmani, and was in charge of all the terrorists' operations in Iraq until he was killed in an American airstrike last November.

He had previously been a colonel in military intelligence and served in Hussein's Republican Guard.

Other senior figures include Abu Ali al-Anbari, who currently acts as Baghdadi's deputy leader in Syria and is a former military general in Hussein's Iraqi Army, and Abu Ayman al-Iraqi, a former colonel in Iraqi Air Force intelligence and now plays a leading role in ISIS' military council.

Less prominent militants with an Iraqi Army backgrounds include Abu Ahmad al-Alwani - a former Baathist soldier and now an ISIS military council member, Abu Kassem - a former Iraqi officer and now in charge of ISIS suicide bombers and foreign fighters.

Another former Iraqi Army colonel was Haji Bakr, who is widely considered to have been al-Baghdadi's closest advisor and the overall head of his military council until his execution at the hands of a rebel group known as the Syrian Martyr's Brigade in January 2014.

It is widely believed that all of the senior ISIS figures killed over the past year - including Haji Bakr - have been replaced by former members of the Iraqi Army.

Horror: Images like this one of the soldiers being led through Tikrit shocked Iraq and the world

This morning Iraqi forensic teams in the newly liberated city of Tikrit have started exhuming bodies from mass graves believed to contain some of the hundreds of soldiers killed by ISIS last year.

Kamil Amin, from Iraq's Human Rights Ministry, said the work on eight locations started yesterday inside Tikrit's complex of presidential palaces, where much of the killing is thought to have occured.

ISIS overran Saddam Hussein's hometown last June, capturing around 1,700 soldiers as they were trying to flee Camp Speicher, an air base previously used by U.S. troops on the outskirt of Tikrit.

The fall of Tikrit was part of the ISIS onslaught that stunned Iraqi security forces and the military, which melted away as the militants advanced and captured key cities and towns in the country's north and west.

Later, ISIS group posted graphic images online that showed gunmen massacring scores of the soldiers after forcing them to lay face-down in a ditch with their arms tied behind their backs.

After weeks of bitter clashes, Iraqi forces and allied Shiite militias, succeeded in retaking Tikrit from ISIS. Their victory was helped by U.S.-led coalition airstrikes.